Min requirement for softmod & can certain Wii's NOT be softmod'd?

I have just softmodded my 4.2e Wii for use as a media streamer. Works perfectly and I would now like to add another 5 to the kids bedrooms ;-) When purchasing secondhand Wii's is there anything I should ask (software version etc) to ensure that I can softmod them? Will there be any that can't be modded for some reason?

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I Prefer the one's made before 2008, as you can install Bootmii as boot2, and they have the early sort DVD drive so you can use it as a DVD player, game ripper and play data discs. I also like them for playing Gamecube games from DVD. You can get several Gamecube games on one disc. The only downside to having the early sort DVD drive is that they can struggle to read dual layer DVDs.